The present invention relates to numerically controlled sheet fabrication centers and/or machines, and particularly relates to an apparatus and method for maximizing the speed of fabricating from a sheet blank, with the dimensional accuracy of the produced parts being maintained above a predetermined threshold.
In a production run of a sheet fabrication machine, such as for example a turret punch press in a flexible manufacturing system (FMS) or a sheet fabrication center, large sheet blanks or worksheets are used. This is due mainly to: (1) the capability provided in currently available software that allows an operator/programmer to program fabrication of multiple parts from a single large worksheet, and (2) the increasing size of sheet fabrication machines such as hydraulic turret punch presses that allows large worksheets to be processed. The programming of multiple different parts to be cut from a single worksheet is moreover made possible with the onset of nesting programs which increase the productivity by increasing the material utilization and reducing the inventory of stock materials needed previously for producing different parts from different sheet blanks, as any part can be nested with other parts at any given time when needed. But the demand for just in time production substantially reduces, if not completely eliminates, the time available for testing the suitability of a particular nesting of parts for a given dimensioned worksheet for fast and accurate production of the different parts from the worksheet. However, the paramount requirement for any production runs, including just in time production, remains the same. That is, parts need to be produced by using the fastest possible speed, and the dimension of each of the produced parts has to be within the required design accuracy.
Yet this requirement to produce accurate parts at the fastest possible speed at which a sheet fabrication machine can operate is frustrated up to now because of the deflections and vibrations that are inherent in the structure of sheet fabrication machines. These inherent deflections and vibrations are caused by the moving components of each sheet fabrication machine such as for example the high torque axes drive systems of a turret punch press. The high accelerations from the axes drive systems are in turn introduced to the coordinate table of the turret punch press which typically is made from steel or a combination of light alloy and steel. When supporting loads, the weight of the loads will further cause deflections or vibrations in the mechanical structure of the sheet fabrication machine and its axes guide system. The deflection causing forces increase with the size of the worktable and the size of the worksheet being introduced to the machine. In a conventional turret punch press, the maximum sheet blank size can be five feet by ten feet, or even greater depending on the dimensions of the worktable onto which the worksheet is placed. With the thickness of each sheet blank typically being up to or sometimes even greater than {fraction (5/16)} inch, substantial forces are loaded onto the mechanical structure of the sheet fabrication machine.
Attempts have been made to strengthen the structure of the sheet fabrication machines so that the amount of structural deflections and vibrations is reduced. However, the design of a non-deflecting mechanical structure for a sheet fabrication machine capable of handling high loads with high positioning speed would be very expensive, if not down right impossible to achieve with present day technology.
Prior to the instant invention, to produce accurate parts from a sheet fabrication machine, it is common practice for an operator or programmer to slow down the table speed of the machine via program codes or commands, or both, input to the manual control panel. In addition, the acceleration and deceleration rates are decreased to reduce forces and vibrations in the sheet fabrication machine. This slowing down of the movement speed and also the acceleration and deceleration rates does produce accurate parts. However, this methodology requires constant interfacing by the operator or programmer of the machine, and is highly dependent on the individual skills of the operator/programmer. Another method previously introduced by Finn-Power to produce accurate parts from a sheet fabrication machine is the use of programmable dwells prior to releasing the punch stroke. This allows a worksheet to stop its vibrations and be positioned correctly prior to being punched. All of the above-noted techniques of producing accurate parts from a worksheet involve the adjustment of various parameters manually, either from the programming station or from the input terminal connected to the computerized numerical control (CNC) panel of the sheet fabrication machine.
To insure that accurate parts are indeed produced by the above-noted techniques, the values used for the different parameters are usually input with a safety margin of error. This however results in the sheet fabrication machine not being used to its fullest potential, thus resulting in longer production time and lesser throughput. Furthermore, depending on the skill of the operator/programmer, even with the input of what are deemed to be xe2x80x9csafexe2x80x9d values for the various parameters, parts produced from the machine oftentimes are below the accepted values and are scraped, and the production run has to be repeated.
To achieve the fastest possible production speed and the highest possible throughput for every conceivable nesting of parts from a sheet blank, each individual positioning move of the sheet blank is independently evaluated based on, but not limited to, a number of parameters such as positioning direction, the distance of the move, the location of a hole to be punched on the sheet blank, the rigidity and the weight of the sheet blank, and the gravity center of the sheet blank relative to where the sheet blank is held by the positioning clamps or other positioning means. Based on a determination of how the various parameters each would affect the accuracy of parts produced from a sheet blank, historical data previously collected is input to the processor of the sheet fabrication system to calculate, among other things, the respective values of the maximum allowable positioning acceleration, deceleration, punch speed and delay prior to the punch stroke for each individual punch stroke. Given the fact that each positioning move is calculated to proceed at the fastest possible speed with the minimum safe positioning time for the required accuracy, the fabrication of parts from the sheet fabrication system is therefore optimized to provide the highest part throughput with the required accuracy.
To enable the processor of the sheet fabrication system to calculate each positioning move optimally, fuzzy logic, either in the form of software or as an add-on module that can be obtained commercially, is provided to the processor of the system. Data relating to the required hole to hole tolerance for a part is input to the processor for each type of sheet blank so that the processor of the system can calculate the optimal positioning move to maximally produce parts from each sheet blank type. Depending on the tolerance required, the processor can instruct the machine to operate at a given pacexe2x80x94setting positioning parameters to effect a xe2x80x9cslowerxe2x80x9d production speed, i.e. less productivity in quantity but better quality, when the tolerance for producing a particular positioning requirement is high, and faster production speed, i.e. higher productivity, when the tolerance requirement is xe2x80x9cloosexe2x80x9d. Furthermore, by providing historical data to the fuzzy logic, the processor of the system is able to learn to make positioning corrections to compensate for normal wear of the machine, the particulars in some areas of the worktable of the machine and other happenstance that relates to the machine.
Since different machines of different models of machines have inherent different characteristics due to the tolerances provided in the manufacture of the machines and their respective coordinate axes motion systems, a static process control (SPC) test can be run to produce data from different loads for the different machine models at different conditions. The produced data is input to the processor of the system, and specifically to the fuzzy logic module, which then uses the data to calculate optimized individual moves for that particular machine. The data may be referred to as compensating data.
The compensating data can be entered continuously, when available from parts produced from a particular machine. The dimensions of each of the produced parts are measured and input to the processor of the system so that the various parameters stored therein for calculating each positioning move are updated for maximum productivity and required accuracy. In place of data measured from parts produced, additional SPC data, which had previously been stored in a data base, could be reused to continuously update the parameters used by the processor of the machine to calculate the positioning moves that are necessary for that machine to produce parts from different types of sheet blanks. The condition of the machine is therefore continuously updated so that the various parameters for each of the positioning moves can be adjusted on a real time basis.